HashHistory is a tied hash module that you interpose
between your program and another tied hash module, usually a DBM
interface. Everything looks completely ordinary, but you can also ask
HashHistory for the history of a key. It will return a list
of all the values that the key has ever had, in order.
For updates and other information, visit
`http://www.plover.com/~mjd/perl/HashHistory/'.
-------------------------------- INSTALLATION INSTRUCTIONS
Unpack the .tar.gz file.
Run:
perl Makefile.PL
make
make test
make install
-------------------------------- MANUAL
NAME
Tie::HashHistory - Track history of all changes to a tied hash
VERSION
This file documents `Tie::HashHistory' version 0.03
SYNOPSIS
my $hh = tie %hash => Tie::HashHistory, PACKAGE, ARGS...;
@values = $hh->history(KEY);
DESCRIPTION
`Tie::HashHistory' interposes itself between your program and another
tied hash. Fetching and storing to the hash looks completely normal, but
`Tie::HashHistory' is keeping a record of all the changes to the each
key, and can Tie::HashHistory will give you a list of all the values the
key has ever had, in chronological order.
The arguments to the `tie' call should be `Tie::HashHistory', and then
the arguments that you *would* have given to `tie' to tie the hash
without the history feature. For example, suppose you wanted to store
your hash data in an NDBM file named `database'. Normally, you would
say:
tie %hash => NDBM_File, 'database', $flags, $mode;
to get this history feature, just add `Tie::HashHistory' before
`NDBM_File':
my $hh = tie %hash => Tie::HashHistory,
NDBM_File, 'database', $flags, $mode;
The data will still be stored in `database', and it will still be an
`NDBM' file. All the fetching and storing will look the same, but the
change history of each key will be available.
The `tie' call will return an object; to find out the history of a key,
use the `history' method on this object. It takes one argument, which is
a key string. It will return a list of all the values that have ever
been associated with the key, in chronological order, starting with the
most recent. For example:
$hash{a} = 'first';
$hash{b} = 'second';
$hash{a} = 'third'; # Overwrites old value
# Prints "third second" as you would expect
print "$hash{a} $hash{b}\n";
@values = $hh->history('a');
# @values now contains ('third', 'first')
@values = $hh->history('b');
# @values now contains ('second')
At present, if called in scalar context, the `history()' method will
return the number of items in the history. This behavior may change in
future versions. The underlying hash can be any tied hash class at all.
To use a regular in-memory hash, use Tie::StdHash (distributed with
Perl) as the underlying implementation:
use Tie::Hash; # *NOT Tie::StdHash*
my $hh = tie %hash => Tie::HashHistory, Tie::StdHash;
This is not as efficient as it could be because fetches and stores on
`%hash' still go through two layers of tieing. I may fix this in a
future release.
Bugs and Caveats
You cannot use `delete' on a `Tie::HashHistory' hash, because it is not
clear yet what it should do. It could revert the value to the previous
version (this would be easy to implement) or it could record in the
history that the key was deleted. (This is more difficult.) A future
version of this package may provide subclasses with one or the other
functionality.
This module needs some more test files.
Author
Mark-Jason Dominus, Plover Systems
Please send questions and other remarks about this software to `mjd-
perl-hashhistory@pobox.com'
For updates, visit `http://www.plover.com/~mjd/perl/HashHistory/'.
Thanks to Randal Schwartz and Chris Nandor for their assistance with the
`**!!**' line.